1. Field of the Invention
The present invention relates to a scanning lens molded from resin and a method of positioning the scanning lens. In particular, the present invention relates to a scanning lens molded from resin which is positioned easily. The present invention also relates to a method of positioning the scanning lens with ease.
2. Description of Related Art
FIG. 12 shows a schematic plan view of a known optical scanning device 100 such as an electronic copier, a laser beam printer, or a laser facsimile machine.
As shown in FIG. 12, optical scanning device 100 is provided with a light source unit 110, a rotary polygonal mirror 114, and a scanning lens 116.
Light source unit 110 is provided with a light source 111, such as a semiconductor laser, and a collimate lens 112. Rotary polygonal mirror 114 scans a laser beam L1 emitted from light source unit 110. Scanning lens 116 focuses laser beam L1 to form an image on a photosensitive body (not shown) on a rotary drum (not shown) arranged outside of an optical box 118 (at an end point of laser beam L1) while the rotary drum is rotating on its axis. Then, electrostatic latent images are formed on the surface of the rotary drum while polygonal mirror 114 and the rotary drum rotate. Scanning lens 116, which is called as a f-theta lens, corrects distortions of an image which is formed on the photosensitive body on the rotary drum. Scanning lens 116 is an axially asymmetrical aspherical lens and is integrally molded with plastic resin such as PMMA, whose chemical name is polymethylmethacrylate (otherwise known as “acrylic”). As would be appreciated by those skilled in the art, scanning lens 116 must be positioned accurately in order to perform as the f-theta lens.
A structure of conventional scanning lens 116 is shown in FIGS. 13 and 14.
When positioning scanning lens 116 relative to optical box 118. scanning lens 116 must be positioned properly in at least three directions: (1) a main-scanning direction X (FIG. 13), which corresponds to a direction of the scanning of laser beam L1, (2) an optical-axis direction Z (FIG. 14), which corresponds to a direction of an optical axis of scanning lens 116, and (3) a sub-scanning direction Y (FIG. 13), which is orthogonal to both main-scanning and optical-axis directions X and Z.
In order to position scanning lens 116 in all three of these directions, scanning lens 116 has a plane 116A orthogonal to optical-axis direction Z and a projection 116B extending in sub-scanning direction Y, provided at the center of main-scanning direction X.
As shown in FIG. 15, when positioning scanning lens 116, an end face 116C of scanning lens 116 is positioned in contact with the bottom face of optical box 118. This permits scanning lens 116 to be positioned in sub-scanning direction Y. To position scanning lens 116 in optical-axis direction Z, plane 116A is placed in contact with a surface of positioning ribs 120 (see FIG. 12) fixed to the bottom face of optical box 118. Finally, projection 116B is fitted into an indentation (not shown) formed on the bottom surface of optical box 118. This permits scanning lens 116 to be positioned in main-scanning direction X.
After the positioning, scanning lens 116 is fixed on the bottom wall of optical box 118 in conventional ways, such as using adhesives or springs.
It is important to position scanning lens 116 in main-scanning direction X, sub-scanning direction Y, and optical-axis direction Z relative to laser beam L1 scanned by rotary polygonal mirror 114 in order to form good images on the photosensitive body.
However, as known to those skilled in the art, the shape of scanning lens 116 may include one or more errors (shape errors), which are introduced during the molding process of scanning lens 116. In detail, as shown in FIG. 16, end face 116C is not manufactured to be parallel with main-scanning direction X because a pressure distribution of the resin in a molding cavity is non-uniform and a temperature of the cavity also is not uniform. These non-uniformities cause the resin to contract in a non-uniform manner. Thus, when end face 116C is aligned with the bottom face of optical box 118, scanning lens 116 will not be parallel with main-scanning direction X. As a result, an image formed on the rotary drum may be distorted.